CMV is one of the viruses which belong to the herpesvirus group and is composed of DNA, core protein, capsid, and envelope. Scarcely any diseases are caused in most cases when a human being is infected by this virus; however, it sometimes causes fatal infections such as hepatitis seen with an infected newborn who has low immunity and interstitial pneumania found with a patient who is immunosuppressed because of organ transplantation. Accordingly, effective means of diagnosis, prevention, and therapy of such infections have been greatly needed in medical circles. Condie et al. (see American J. Medicine, March 30, 1984, pp. 134-141) report successful cases where administration of human serum immunoglobulin with a high antibody titer to CMV protected bone marrow transplant recipients from CMV infections and interstitial penumonia arising from the same. Human serum immunoglobulin with a high titer is prepared by collecting and fractionating high-titer blood plasma only, the selection of which is made by checking the antibody titer of the serum of donors beforehand. The antibody titer to CMV of serum immunoglobulin thus prepared is at the most 10 times that of serum immunoglobulin prepared from blood plasma collected randomly and collection of such high-titer serum immunoglobulin as this is very difficult to make in sufficient amounts to ensure a stabilized supply.
The production of high-purity antibody, or MCA, has been made possible since the establishment of the hybridoma method by Milstein and Kohler. Rasmussen et al. (see Proc. Natl. Acad. Sci., U.S.A., 81, pp. 876-880, 1984) provided hybridomas capable of producing MCA specific to CMV by fusing mouse myeloma cells and spleen cells of mice immunized with CMV. Several other research groups have obtained mouse MCA specific to CMV (for instance, Goldstein et al., in Infection and Immunity, 38, pp. 273-281, 1982; Pereira et al., in Infection and Immunity, 36, pp. 92414 932, 1982). Of these MCA, few MCA are seen to have enough active capacity to neutralize viruses; however, MCA provided by Rasmussen et al. has a capacity to neutralize CMV at about 10 .mu.g/ml. This neutralizing capacity is very high when compared with high-titer serum immunoglobulin and is expected to be of use for the prevention or remedy of CMV infections. But, being derived from mice, these MCA are recognized as a foreign substance upon administration to humans, thus causing harmful side effects. Therefore, the development of anti-CMV MCA, arising from humans instead of mice, is hoped for.
Human MCA are generally produced from hybridomas obtained by cell fusion between mouse myeloma cells, human myeloma cells, or cells established from other lymphoid tissues and human lymphocytes. They are also produced from lymphoblast cells obtained by transforming human lymphocytes with Epstein-Barr virus. Many attempts have been made since 1980 to develop human MCA but every method has had its own characteristic problem. MCA produced by a hybridoma obtained by cell fusion of mouse myeloma cells with human lymphocytes is not stable, while the cell fusion between human myeloma cells and human lymphocytes is very low in efficiency. Production of MCA from lymphoblast cells obtained by means of EB virus involves problems of low yield and instability. Moreover, EB virus is capable of causing tumors, thus raising a serious problem of safety. The techniques of establishing MCA-producing cells involves great difficulties as described the above; however, there is another serious barrier, we find lying in the means of obtaining MCA specific to specialantigens. It is a problem generally seen in collecting lymphocytes from fully immunized humans. Generally speaking, lymphocytes of normal persons are in many cases sensitized by CMV but its degree of immunity is very low. Therefore, even when hybridomas or lymphoblast cells are established from lymphocytes of normal persons, it is hardly expected to obtain cells producible of anti-CMV MCA.
With regard to human MCA against CMV, a report is made on a single case of establishing a cell line producing the same (see J. Immunol., 133, pp. 2202-2205). According to this report, a MCA-producing cell line is obtained by transforming lymphocytes of normal persons with EB virus. The report, however, contains only one photograph of fluorescence antibody and it is not made clear whether a cell line which produces MCA with secured stability is established or not. Morever, it also is accompanied by the problem of carcinogenecity arising from EB virus as mentioned before and it is apprehended that it may involve a great risk of life upon its in vivo administration. It is also made apparent that this MCA is incapable of neutralizing CMV even if it binds to CMV.
As explained in the above, the hardest problem encountered in efficiently obtaining desired specific human MCA is the difficulty in sufficiently immunizing human lymphocytes with a specific antigen. In a case where mice are used, even an antigen which is harmful in vivo can be given to them and immunization can also be carried out according to a schedule conveniently suited for the purpose. This is impracticable in the case of human beings, since CMV is a pathogen and so far as no vaccine has yet been developed. Thus, it is not allowed morally to give CMV to human beings intentionally for the purpose of immunization. This is another problem that confronts the production of human MCA stably. Both the hybridoma method and the EB virus method have merits and demerits as described in the above.